Re: [RFC PATCH 1/3] RDMA/umem: Change for rdma devices has not dma device

[Jason Gunthorpe]

On Wed, Sep 08, 2021 at 09:22:37PM +0200, Daniel Vetter wrote:
> On Wed, Sep 8, 2021 at 3:33 PM Christian König <christian.koenig@xxxxxxx> wrote:
> > Am 08.09.21 um 13:18 schrieb Jason Gunthorpe:
> > > On Wed, Sep 08, 2021 at 05:41:39PM +0900, Shunsuke Mie wrote:
> > >> 2021年9月8日(水) 16:20 Christoph Hellwig <hch@xxxxxxxxxxxxx>:
> > >>> On Wed, Sep 08, 2021 at 04:01:14PM +0900, Shunsuke Mie wrote:
> > >>>> Thank you for your comment.
> > >>>>> On Wed, Sep 08, 2021 at 03:16:09PM +0900, Shunsuke Mie wrote:
> > >>>>>> To share memory space using dma-buf, a API of the dma-buf requires dma
> > >>>>>> device, but devices such as rxe do not have a dma device. For those case,
> > >>>>>> change to specify a device of struct ib instead of the dma device.
> > >>>>> So if dma-buf doesn't actually need a device to dma map why do we ever
> > >>>>> pass the dma_device here?  Something does not add up.
> > >>>> As described in the dma-buf api guide [1], the dma_device is used by dma-buf
> > >>>> exporter to know the device buffer constraints of importer.
> > >>>> [1] https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flwn.net%2FArticles%2F489703%2F&amp;data=04%7C01%7Cchristian.koenig%40amd.com%7C4d18470a94df4ed24c8108d972ba5591%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637666967356417448%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C2000&amp;sdata=ARwQyo%2BCjMohaNbyREofToHIj2bndL5L0HaU9cOrYq4%3D&amp;reserved=0
> > >>> Which means for rxe you'd also have to pass the one for the underlying
> > >>> net device.
> > >> I thought of that way too. In that case, the memory region is constrained by the
> > >> net device, but rxe driver copies data using CPU. To avoid the constraints, I
> > >> decided to use the ib device.
> > > Well, that is the whole problem.
> > >
> > > We can't mix the dmabuf stuff people are doing that doesn't fill in
> > > the CPU pages in the SGL with RXE - it is simply impossible as things
> > > currently are for RXE to acess this non-struct page memory.
> >
> > Yeah, agree that doesn't make much sense.
> >
> > When you want to access the data with the CPU then why do you want to
> > use DMA-buf in the first place?
> >
> > Please keep in mind that there is work ongoing to replace the sg table
> > with an DMA address array and so make the underlying struct page
> > inaccessible for importers.
> 
> Also if you do have a dma-buf, you can just dma_buf_vmap() the buffer
> for cpu access. Which intentionally does not require any device. No
> idea why there's a dma_buf_attach involved. Now not all exporters
> support this, but that's fixable, and you must call
> dma_buf_begin/end_cpu_access for cache management if the allocation
> isn't cpu coherent. But it's all there, no need to apply hacks of
> allowing a wrong device or other fun things.

Can rxe leave the vmap in place potentially forever?

Jason